High temperature superconductor (HTS) filters have applications in telecommunication, instrumentation and military equipment. The HTS filters have the advantages of extremely low in-band insertion loss, high out-of-band rejection and steep skirts due to the extremely low loss in the HTS materials. In a typical design, HTS filters and mini-multiplexers are comprised of spiral resonators.
Filter performance is highly dependent on the frequencies of the resonators of which the filter was comprised. In turn, variations in circuit parameters such as resonator patterning, substrate thickness and dielectric constant, and HTS material properties affect the frequency of the resonators. It is both difficult and costly to try to control these parameters precisely during production. The difficulty in producing the desired HTS filter pattern increases as the number of resonators or poles in the filter increases.
It is therefore desirable to tune the filters after they have been produced. One approach to tuning the center frequency of such a filter proposed by Shen, WO 01/41251, involves providing a plate spaced a distance apart from and opposite to the HTS filter. The plate contains a conductive film, preferably a HTS film, on at least a portion of the surface of the plate that faces the filter. The distance between the plate and the filter can be adjusted to tune the center frequency of the filter. Alternatively, the individual resonators of the filter can be mechanically tuned by adjusting a screw or a dielectric rod. However, since the resonators generally vary in a random fashion, each pole of the filter must be individually tuned and the tuning of each pole affects every other pole in the filter. The tuning process can typically take hours to perform.
Humphreys, U.S. Ser. No. 03/048,148, and N. J. Parker et al, 2000 IEEE MTT-S, page 1971, disclose tuning a microwave or RF circuit by directing a laser beam onto the microwave or RF circuit so as to alter the material properties of selected areas of the microwave or RF circuit. Results are disclosed for a simple microstrip λ/2 resonator and a pseudo-elliptic filter comprised of 3 simple microstrip λ/2 resonators. They disclose that it is easier to increase the resonator frequencies than to reduce them.
An object of the present invention is to tune the resonance frequency of a HTS spiral resonator or a HTS coil and to improve the production yield of HTS filters comprised of HTS spiral resonators by providing a method for tuning the characteristics of such a filter by tuning individual HTS spiral resonators.